1. Field of the Invention
The present invention relates to a Base Station (BS) and method of the BS for controlling a Mobile Station (MS). More particularly, the present invention relates to a BS and a method of the BS for controlling the scanning operation of an MS for avoiding problems caused by coexistence of MS-initiated and BS-initiated scanning attempts.
2. Description of the Related Art
In the Mo-Worldwide Interoperability for Microwave Access (M-WiMAX) of the related art, frequency scanning of an MS is performed to discover at least one target handover cell among neighbor cells using frequencies different from that of the serving cell. That is, once scan trigger conditions are informed by a Type, Length, Value (TLV) field in a Downlink Channel Descriptor (DCD) message broadcast by a BS, the MS transmits a mobile scan request (MOB_SCN-REQ) message when the scan trigger conditions are fulfilled. Afterward, the MS is assigned a scan duration by means of a mobile scan response (MOB_SCN_RSP) message and collects information about neighbor cells via a scanning process. The scan trigger conditions include Received Signal Strength Indication (RSSI) and Carrier to Interference plus Noise Ratio (CINR). The frequency scan triggered by the request from the MS is referred to as an MS-initiated scan.
As the need for improvement of a WiMAX network has increased gradually, enhanced technologies such as Load Balancing (LB), Inter-Cell Interference Coordination (ICIC), Location-Based Service (LBS), and Self-Organizing Network (SON) have been adopted. Most of these technologies can be executed with the exchange of information such as RSSI, CINR, and Relative Delay between a serving BS and neighboring BSs. The serving BS transmits the MOB_SCN_RSP message, without request from the MS, to trigger the MS to carry out scanning and acquires the information necessary from a mobile scan report (MOB_SCN_REP) message transmitted by the MS after a predetermined time. This is referred to as a BS-initiated scan or an unsolicited scan.
While the MS-initiated scan is in progress, the MOB_SCN_RSP message for triggering the BS-initiated scan may be received during the interleaving interval. In this case, the MS starts the BS-initiated scan based on the scan parameters received without completion of the MS-initiated scan. This problem also can occur when the scan trigger conditions indicated by TLV of the DCD message are fulfilled during the interleaving interval in the progress of BS-initiated scan. In this case, the MS transmits the MOB_SCN-REQ message to the BS and the BS replies by transmitting the MOB_SCN_RSP message to the MS such that the MS starts the MS-initiated scan based on the scan parameter received newly in the state before the completion of the previous BS-initiated scan. Such a scan timing conflict causes scanning errors, resulting in performance degradation.